This invention relates to an apparatus for, and a method of, measuring thermal conductivity (.lambda.) and heat capacity (C) of fluids, and more particularly, to an excitation/relaxation calorimeter which measures thermal conductivity and heat capacity of a fluid simultaneously.
Measurement of the thermal properties, e.g. thermal conductivity, heat capacity and thermal diffusivity, of materials, including biomaterials, is essential to being able to understand and use them. For example, recent studies of the properties of phospholipid dispersions in water, indicate that a spontaneous transformation from the multilamellar liquid-crystal state to a suspension consisting only of large unilamellar vesicles will occur when the ambient temperature of the dispersion is increased to the transformation temperature. The unilamellar vesicles which form are considered to be a critical state and the thermodynamic properties of the critical unilamellar state indicate the transformation occurs without a latent heat. Such properties have been inferred primarily from measurements of the air/water surface films in equilibrium with the aqueous phospholipid dispersion. There is a need to measure the temperature dependence of the heat capacity of the lipid dispersions directly.
Differential scanning calorimeters, calorimeters which measure the temperature difference between a sample fluid and a reference fluid in response to a temperature scan, have been widely used to investigate the macroscopic properties of matter. However, commercial differential scanning calorimeters have been unable to make such measurements due to the additional constraints inherent in biomaterials in aqueous solutions. Such constraints include irreproducibility of the baseline, the small sample size available (e.g. milligram quantities of membrane materials), filling errors including artifacts from trapped air in the sample chamber, the chamber to sample mass ratio needed to maximize signal sensitivity and a measuring sensitivity of the order of, for example, 400 .mu.J/.degree. C..multidot.cm.sup.3.
C. P. Mudd et al., "A differential heat-conduction microcalorimeter for heat-capacity measurements of fluids," Journal of Biochemical and Biophysical Methods, 26 (1993) 149-171, describe a calorimeter with high sensitivity for measuring the specific heats of fluids. However, that calorimeter does not measure thermal conductivity and heat capacity simultaneously; rather it measures thermal diffusivity (thermal conductivity is known and approximately constant) and calculates heat capacity from the thermal diffusivity relationship: EQU .eta.=.lambda./C.sub.p,
where .eta. is thermal diffusivity, .lambda. is thermal diffusivity and C.sub.p is the volume heat capacity. Mudd et al.'s calorimeter was designed, in part, because of their discovery that the peak of the response to a brief thermal pulse was approximately four times more sensitive to heat capacity than the thermal conductivity of the aqueous lipid dispersions. By limiting their measurements to a narrow range of fluids, with heat capacities near that of the dilute lipid dispersions, and with similar diffusivities, they were able to circumvent approximating the effect of .lambda., which in principle may change independently of heat capacity.
Although Mudd et al. avoided measuring .lambda. in lipid dispersions, such an assumption cannot be made for all biomaterials. Furthermore, in addition to the relationship between thermal conductivity and heat capacity for a given material, the interplay between heat capacity and thermal conductivity in almost all thermal instruments is a well recognized problem which can affect measurements of the material's properties. Previous efforts in dealing with this problem involve two approaches. In the first approach, the effect of one of the properties is minimized, then the other property is measured. In the second approach, both properties are measured simultaneously. Examples of such approaches can be found in D. Bertolini et al., "A differential calorimetric technique for heat capacity and thermal conductivity measurements of liquids," Rev. Sci. Instrum. 61(9), September 1990, 2416-2419, A. Bernasccni et al., "Dynamic technique for measurement of the thermal conductivity and the specific heat: Application to silica aerogels," Rev. Sci. Instrum. 61(9), September 1990, 2420-2426 and N. Gershfeld et al., "Critical Temperature for Unilamellar Vesicle Formation in Dimyristoylphosphatidylcholine Dispersions from Specific Heat Measurements," Biophysical Journal, 65, September 1993, 1174-1179. Both approaches suffer from limitations of applicable materials, lack of precision or both.
Therefore, it is an object of the invention to provide a calorimeter which measures thermal conductivity and heat capacity simultaneously. It is another object of the invention to provide a calorimeter which minimizes the effects of the thermal conductivity and heat capacity of components of the measurement instrument.